Correct way to inherit from Abstract class with dependent interfaces in C#

I have an abstract class for sending and retrieving messages.

public abstract class MailClient
{
    public IAuthentication MailAuthentication { get; set; }

    internal MailClient(IAuthentication mailAuthenticaton)
    {
        this.MailAuthentication = mailAuthenticaton;
    }

    public abstract State SendMessage(IMessage message);
    public abstract List<IMessage> GetEmails();
}

I want to create a concrete class (let say for yahoo email messages). So, I create yahoo client that inherits from the abstract Mail client, and uses YahooMessage object that contains details for the message that needs to be sent or received.

public class YahooClient : MailClient
{
    private YahooConfiguration configuration = new YahooConfiguration();

    public YahooClient (string username, string password) : base(new YahooAuthentication(username, password)) 
    { 
    }

    public override List<YahooMessage> GetMessages()
    {
        //Code for retrieving emails
    }

    public override State SendMessage(YahooMessage message)
    {
        //Code for sending emails
    }
}

YahooMessage implements the IMessage interface, and adds few new properties that are specific for Yahoo.

However, I got error because "SendMessage" and "GetMessages" are not implemented with the correct signature in the Child class (YahooClient). Instead of IMessage I use YahooMessage, that implements the IMessage interface.

This is obviously the wrong approach. What would be the suitable approach to achieve the desired functionality?